A solvent deasphalting unit associated with an oil refinery mixes residual oil produced by a petroleum refinery with a light hydrocarbon solvent such as propane, iso- or normal butane, iso- or normal pentane, or mixtures thereof, for producing two liquid product streams. One stream is substantially free of asphaltenes and contains deasphalted oil (DAO) and solvent, and the other stream contains asphaltene and solvent within which some DAO is dissolved. These product streams are applied to a solvent recovery section which extracts most of the solvent from the product streams. The resultant solvent-free DAO is returned to the refinery for conversion to gasoline, jet fuel, etc.; and the resultant solvent-free asphaltene can be combined with dilutent, such as diesel fuel, for conversion to residual fuel.
In some installations, the solvent recovery section includes a supercritical solvent recovery section that removes a large percentage of solvent from the product streams, followed by an evaporative solvent recovery section that removes the balance of solvent. In other installations, only an evaporative solvent recovery section is used. In both cases, the output of the evaporative solvent recovery section is DAO product and asphaltene product having acceptable levels of solvent (e.g., 0.05% by weight).
In an evaporative solvent recovery section, each of the liquid product streams of DAO and solvent, or asphaltene and solvent is first flashed to produce a vaporized solvent stream, and a reduced solvent liquid product stream. Each of the reduced solvent liquid product streams so produced are then subjected to serial flashing and/or stripping until the final product stream is free of solvent to the desired degree. The vaporized solvent produced in this manner is condensed and re-used.
In order to reduce the amount of heat lost as a result of the condensation of the vaporized solvent, the temperature at which flashing operations are effected is kept as low as possible. Thus, the flash drums to which the solvent containing product streams are applied operate to produce solvent vapor at about 220.degree. F. Heat contained in these vapors is of such low quality that economic recovery is not practical; and as a consequence, such heat is extracted from the solvent by air or water cooling, and is lost to the environment.
If the temperature of a product stream applied to the lead flash drum of an evaporative solvent recovery section is less than about 250-300.degree. F., which will effect the production of vaporized solvent at the desired temperature of about 220.degree. F., heat must be added to the product stream before flashing. All of the added heat contained in the solvent that flashes in the drum will be lost to the environment. After flashing is effected, the reduced solvent product stream extracted from the bottom of the drum eventually is applied to a stripper which must operate at a temperature high enough to ensure that only a minimum amount of solvent is retained in the final product stream. For example, if the desired residual solvent in the final product is to be less than about 0.05% by weight, the stripper must operate at about 525.degree. F. to ensure vaporization of the solvent. Since the temperature of the reduced solvent product stream leaving the preceding flash drum is about 250-300.degree. F., heat must be added to the product stream between a succeeding stripper and a preceding flash drum. Again, most of this added heat is lost to the environment when the vaporized solvent produced by the stripper is condensed.
U.S. patent application Ser. No. 08/572,185, filed Dec. 13, 1995 (the disclosure of which is hereby incorporated by reference) discloses a method of and apparatus for recovering heat contained in the vaporized solvent produced by the first stage of flashing by expanding the vaporized solvent in an organic vapor turbine. This application also discloses recovery of heat in the vaporized solvent produced by the stripping stage by transferring heat to an organic fluid that constitutes the working fluid of an organic vapor turbine that operates on the Rankine cycle.
This approach reduces the heat consumption of an evaporative solvent recover section of a solvent deasphalting unit. An object of the present invention is to provide a method of and means for reducing even further the net heat used in an evaporative solvent recovery section of a solvent deasphalting unit.